The present invention pertains to the technical field of aqueous suspensions of mineral materials involved in manufacturing sheets of paper.
When manufacturing a coated sheet of paper, a first step consists in converting the papermaking pulp, by means of the papermaking machine, into a sheet of paper, which is not yet coated. The papermaking pulp essentially contains natural cellulose fibers or synthetic fibers, water, and one or more mineral materials such as calcium carbonate, as well as various other additives such as so-called “pasting” agents. A mineral material (such as calcium carbonate) used in such situations is referred to as a “filler”. The second step resides in coating the previously obtained sheet. This operation consists in applying to the base paper medium an aqueous compound known as a “paper coating”, which particularly contains water, one or more mineral materials such as calcium carbonate, one or more binders, and various additives. When manufacturing a paper coating, a mineral material (such as calcium carbonate) that is used is referred to as a coating pigment.
After being applied to the medium, the paper coating has a natural tendency to transfer some or all of the water-soluble or suspended substances that it contains into the medium. The slowing down of this migration into the paper medium is necessary in order to maintain a homogeneous distribution of water-soluble or suspended substances within the thickness of the paper coating thereby applied, which ultimately improves the paper's surface condition and printability.
In seeking to resolve the particular technical problem of slowing the penetration of a paper coating into a sheet of paper, though without preventing it from occurring, with the goal being to maintain a homogeneous distribution of water-soluble or suspended substances within the thickness of the paper coating thereby applied, persons skilled in the art have identified two possible technologies:                the first one consists of modifying the rheology of the paper coating by adding to it rheology-modifying agents known as “thickening agents”, which, by increasing the paper coating's viscosity, reduce its penetration into the sheet of paper. The paper coating's “water retention” is referred to in such cases. The problem with this technology is that any significant change to the paper coating's rheology may have negative effects on the process's other parameters (such as pumping, filtering, and spreading the paper coating),        the second one consists of modifying the paper medium's absorbent properties, either by reducing its porosity or by increasing its degree of hydrophobicity.        
There are a number of documents dealing with the problem of maximizing a sheet of paper's hydrophobicity with respect to water (as may be desired for paper which must withstand particularly high humidity conditions) or with respect to ink (as may notably be sought for paper intended to be printed with water-based, and particularly acidic, inks). These documents therefore focus on the penetration of liquids such as water or inks whose formula differs from that of a paper coating. Additionally, they generally rely on extremely simple tests, which involve measuring the time it takes for water or an ink to pass through a sheet of paper. Such tests would be wholly unsuitable at highlighting the technical problem covered by the present application:                firstly, in these tests, the liquid (water or ink) passes through the sheet of paper completely, which is not the case for a paper coating that only partially penetrates the paper medium at the time it is applied thereto;        secondly, these tests do not make it possible to dynamically determine the penetrating speed of the paper coating into the sheet of paper during the first tenths of a second following said paper coating's coming into contact with the paper medium.        
As explained above, only the technology implemented in the tests given herein can provide an understanding of the phenomena described hereinabove.
Consequently, the documents dealing with the problem of maximizing the hydrophobicity of a sheet of paper with respect to water or ink therefore solve very different problems than the one which forms the object of the present application. If the Applicant mentions these documents anyway, this is done merely to demonstrate that the person skilled in the art, even if he had chosen to refer to said documents (which no objective elements could lead him to consider, because said documents do not treat the technical problem that he wishes to solve), would have found no information disclosing or suggesting the technical solution which is the object of the present application.
Thus, in order to increase the hydrophobicity of a sheet of paper with respect to water or ink, the person skilled in the art is familiar with several solutions, which may be divided into three categories during their use in manufacturing a sheet of paper:                a particular additive, with properties that limit water penetration into said sheet,        pasting agents that grant the paper medium's cellulose fibers, which are naturally hydrophilic, hydrophobic properties that improve their resistance to water,        mineral materials such as calcium carbonate, said materials being treated, dispersed, or mixed with a particular organic polymer, said materials thereby treated, dispersed or mixed with said polymer granting the sheet of paper water-penetration resistance properties.        
In the first category, the person skilled in the art is familiar with the document JP 06-219038, which describes a paper containing pulp, a mineral material such as calcium carbonate, and a “penetration-inhibiting” agent that inhibits the penetration of ink and therefore of water-soluble substance. Such an agent is particularly chosen from among hydrophilic resins such as casein and starch, as well as from among copolymers which have both a hydrophilic part and a hydrophobic part, such as copolymers of maleic anhydride and styrene.
In this category, he is also familiar with the document SU 1,263,739, which describes the usage of an agent that makes the sheet of paper hydrophobic, such as a wax, in combination with a mineral material suspension (such as calcium carbonate) and papermaking pulp, in a method for manufacturing a sheet of paper.
The second category is made up of solutions which use pasting agents that are hydrophobic in nature.
As stated in document WO 96/23105, conventional pasting agents of the type which are resins with aluminum sulfates, or those which are based on ketene dimers, exhibit hydrophobic properties which will limit the penetration of water and water-soluble substances into the paper within which said pasting agents are used. This document even discloses the combination of using these pasting agents with a colloidal polymer, for the purposes of improving the hydrophobicity of the sheet of paper
Finally, the third category of solutions relies upon the implementation of a mineral material such as calcium carbonate, said material being treated or mixed with an organic polymer.
In this category, the person skilled in the art is familiar with the document U.S. Pat. No. 5,514,212, which describes the treatment of the surface of a calcium carbonate through precipitation in the presence of a compound of starch and fatty acids having 16 to 18 carbon atoms. A calcium carbonate modified in this manner is used as an aqueous suspension in manufacturing a sheet of paper. This document indicates that the quantity of pasting agents used in manufacturing a sheet of paper may thereby be reduced. It also mentions that the sheet of paper's degree of hydrophobicity is thereby improved. This level of hydrophobicity is determined using acidic water-based ink penetration measurements: this is a test well known to the person skilled in the art as a “Hercules Sizing Test” or “HST”, consisting of determining the time needed for a standardized water-based ink containing formic acid to pass through a sheet of paper. The objective of this test is therefore to determine a paper's resistance to water-based, acidic inks which are intended to be printed upon said paper afterwards. Additionally, the examples given in this document only pertain to uncoated sheets of paper. Consequently, there is no information in document U.S. Pat. No. 5,514,212 that reveals or suggests that the use of calcium carbonates treated using the method described therein slows the penetration of a paper coating into the sheet of paper and leads to an even distribution of water-soluble or suspended substances throughout the thickness of the paper coating applied to the paper.
The person skilled in the art is also familiar with the document WO 01/86067, which describes a method for treating a mineral material such as calcium carbonate using an acrylate-, acrylonitrile- and styrene-based hydrophobic polymer, polymerized in the presence of starch, polysaccharide, or carboxymethylcellulose. Said polymer is mixed with said mineral material in the form of an aqueous solution or suspension, with this solution or suspension later being used in the manufacturing of a sheet of paper, whose degree of hydrophobicity is thereby improved. As with the previous document, the examples do not pertain to coated sheets of paper. Furthermore, these examples consist of rudimentary HST tests.
Finally, the person skilled in the art is also familiar with the document WO 00/03093, which describes a method for manufacturing a sheet of paper, using a mineral material such as calcium carbonate, in the form of an aqueous suspension further containing a copolymer combining acrylonitrile with another monomer such as styrene, with this copolymer giving the calcium carbonate a hydrophobic function. In this document, it is indicated that the calcium carbonate and the copolymer are mixed in the water. The calcium carbonate thereby used in manufacturing the sheet of paper grants said sheet improved hydrophobicity. As with the two previous documents, the examples consist of simple ink-penetration measurements using the HST test. Additionally, the sheets of paper are uncoated.
Consequently, none of these documents reveals or suggests using, in a method for manufacturing a sheet of paper, an aqueous suspension or dispersion of mineral materials as an agent for slowing the penetration of a paper coating into said sheet, said dispersion and/or suspension characterized in that it contains at least one water-soluble copolymer made up of:                a) at least one ethylenically-unsaturated anionic monomer,        b) at least one ethylenically-unsaturated oxylalkylated monomer, terminating in a hydrophobic chain.        
One of the Applicant's innovations particularly relies on the fact that she has noted that mineral materials,                both in an aqueous suspension and/or dispersion,        and in association with a particular copolymer,are particularly suitable for solving the technical problem covered by the present application.        
Without wishing to bound to any theory whatsoever, the Applicant thinks that the mineral material, particularly when it is ground and/or dispersed with said copolymer, then when it is used in manufacturing the sheet of paper, will develop properties of slowing the penetration of a paper coating into said sheet: this property is particularly illustrated in the tests herein, using an original technique of tracking the penetration of the paper coating into the sheet by means of ultrasound, during the first tenths of a second after the paper coating has come into contact with the paper medium. It is thereby demonstrated that, entirely advantageously, the present invention leads to the slowing of the penetration of the paper coating into the sheet of paper, which results in an even distribution of water-soluble or suspended substances throughout the thickness of the paper coating applied to the paper.
The Applicant wishes to indicate that she is, however, familiar with a number of documents dealing with aqueous suspensions of mineral materials which are treated, dispersed, or ground, particularly with water-soluble polymers comprising a hydrophobic group, and used as fillers, i.e. in the manufacturing of a sheet of paper. By indicating such documents, she intends to emphasize that they do not in any way pertain to the technical problem covered herein; to that end, the person skilled in the art cannot find therein any objective instruction that would lead him to the technical solution which is the object hereof.
Thus, with respect to mineral materials used as fillers, said materials being treated, the person skilled in the art is familiar with the document JP 2003-166195. This document describes the treatment of a mineral material used in an aqueous suspension in manufacturing paper, using an organic compound mixed with said mineral material. This organic compound has both a hydrophilic portion and a hydrophobic portion, said hydrophobic portion being made up of an alkyl or alkenyl group having 6 to 24 carbon atoms. The use of an aqueous suspension of mineral materials thereby treated in the manufacturing of paper leads to an increase in the sheet of paper's thickness, which is a very different problem from the one solved by the present invention.
With respect to mineral materials dispersed in an aqueous medium and used as fillers, the person skilled in the art is familiar with the document U.S. Pat. No. 4,801,354. This document describes the use, as a mineral material dispersing agent, of a copolymer having a “Fikentscher K value” from 10 to 65 and made up of a (meth)acrylic ester and an ethylenically-unsaturated carboxylic acid, preferentially isobutylacrylate, whose water solubility is very low (0.2 g in 100 g of water). The aqueous suspensions of mineral materials thereby dispersed (such as calcium carbonate) are used in manufacturing the sheet of paper, where they make it possible to limit the quantity of mineral load retaining agents, which the present invention does not seek to solve.
Finally, with respect to mineral materials dispersed or ground in an aqueous medium, the person skilled in the art is familiar with the document EP 0,892,020. This document describes the use of a copolymer as a dispersing agent and/or grinding aid agent for mineral materials in an aqueous suspension, characterized in that said copolymer is a copolymer with a specific viscosity no greater than 50 and made up of at least one ethylenically-unsaturated monomer with a carboxylic function and at least one ethylenically-unsaturated oxylalkylated monomer terminating in a hydrophobic chain.
The technical problem solved by this document is the search for a polymer that may be used as both a mineral material dispersing and/or grinding agent, no matter whether such materials are hydrophilic, such as calcium carbonate, or hydrophobic, such as talc. Although document EP 0,892,020 indicates that the mineral materials thereby obtained may be used as fillers, there are no examples concerning such usage. Moreover, there is no information in that document revealing or suggesting the use of mineral materials ground in an aqueous suspension and used as fillers with the particular function of slowing the penetration of a paper coating into the sheet of paper.